1. Technical Field
The present invention relates to an improvement in the field of the electrocoagulation printing. More particularly, the invention relates to an electrocoagulation printing ink capable of enhancing a gradation reproducibility and preventing an undesirable background in a non-image region of the print, and a printed matter as well as an electrocoagulation printing method using the ink.
2. Background Art
The electrocoagulation printing is a printing method for carrying out a digital image printing to directly produce a print from a computer without using any printing plates. Some of the printing methods and apparatuses are disclosed in U.S. Pat. Nos. 4,895,629, 5,538,601, and 5,693,206.
In this printing method, after the surface of a positive electrode is cleaned and coated with a coating agent containing an oily substance, a plurality of dots of coagulated ink representing a desired image are produced on the surface of the positive electrode by electrocoagulation of the electrocoagulation printing ink which contains an electrolytically coagulable polymer. After removing any remaining non-coagulated ink from the surface of the positive electrode, the dots of coagulated ink are transferred to a substrate to be printed so as to imprint the substrate with the image.
The expression xe2x80x9celectrocoagulation of the inkxe2x80x9d in the electrocoagulation printing as used herein refers to a process which involves an electrical energizing between the positive and negative electrodes to cause a breakdown of passive oxide film on the surface of the positive electrode, thereby to elute multivalent metal ions from the surface of the positive electrode, and allowing the multivalent metal ions to react with the electrolytically coagulable polymer contained in the electrocoagulation printing ink so as to coagulate the ink. This forms dots of coagulated ink on the surface of the positive electrode. More particularly, the multivalent metal ions released from the positive electrode upon an electrical energizing form ion bonds and/or coordinate bonds with polymer molecules so as to crosslink the polymer molecules one another, so that a coagulum (dots) which is hardly soluble in water is formed.
It is requested for a print produced by the electrocoagulation printing as well as common offset or gravure printing to have a suitable degree of the gradation reproducibility (the expression of a gradation) showing smoothness from brightness to darkness and a practically acceptable level of the image density.
The gradation reproducibility is determined how many amount of the ink is electrolytically coagulated corresponding to a length of the electrical energizing time. The coagulability of the electrocoagulation printing ink corresponding to a positive electrode energizing rate (an amount of electric currents conducted to the positive electrode) is closely related with the degree of the gradation reproducibility. It is therefore focused on the linear response of electrocoagulability to the amount of the multivalent metal ions released from the positive electrode corresponding to the positive electrode energizing rate.
However, it is true that the amount of the multivalent metal ions is too small to be actually measured and therefore the relationship between the electrocoagulability of the electrocoagulation printing ink and the gradation reproducibility on the print remains unclear. Accordingly, it is difficult to propose any electrocoagulation printing ink improved in the gradation reproducibility.
On the other hand, the printing image density on a print which is in general affected by the concentration of a pigment contained in the ink is largely dependent on the size of dots produced by the electrocoagulation. The size of the dots is also dependent on the magnitude of the electrocoagulability corresponding to the amount of the multivalent metal ions released from the positive electrode. Increasing the image density by enlarging the size of the dots hardly improve the expression between brightness and darkness or the gradation reproducibility. It is therefore a significant issue to provide an improved ink balanced well between both the characteristics.
In addition, there is another requirement for improving the removal of non-coagulated ink. More specifically, after the electrocoagulation of a printing ink, a step of removing non-coagulated ink is conducted where the non-coagulated ink is removed from the surface of the positive electrode by a known manner, for example, scraping the surface with a soft rubber squeegee and then, the coagulated ink is transferred as a desired image onto a substrate to be printed. If the rigidity of the squeegee or its pressure against the surface of the positive electrode is increased in order to completely remove (or scrape off) the non-coagulated ink from the surface of the positive electrode, the dots of coagulated ink may partially be dropped out or injured. This may cause the declination of the image density or the generation of white streaks, and thereby decreasing the printing image sharpness, so that the quality of a print may be impaired. When the removal of the non-coagulated ink with the squeegee is failed, bits of the non-coagulated ink remain in the non-image region and may cause the undesirable background. It is a troublesome task to remove the non-coagulated ink to such an extent that an undesirable background on the print is not formed, by controlling the squeegee such that the dots of coagulated ink are not injured. Accordingly, there has been desired to develop an improved electrocoagulation printing ink of which non-coagulated portions of the ink can easily be removed from the surface of the positive electrode.
An another issue is that since the density of an image is controlled largely by an amount of currents supplied between the positive and negative electrodes, there are cases in which the maximum image density obtained using the conventional electrocoagulation printing ink is insufficient depending on normally used amount of the currents. It may be possible to obtain the image with a high density by increasing the electric conductivity of the electrocoagulation printing ink or the voltage applied between the positive and negative electrodes. However, heating the electrodes or excessively increasing the voltage may generate overloads to the relevant electric circuits. Accordingly, as the amount of the currents to be used has to be substantially limited, there are cases in which a deficiency in the image density obtained in practice may hardly be prevented.
It is therefore an object of the present invention to provide an electrocoagulation printing ink in which a level of the electrocoagulability can be changed according to the energizing rate, so that both the gradation reproducibility and the printing image density on a print can be improved, a printed matter printed with the ink and high of quality, and an electrocoagulation printing method employing the ink.
It is another object of the present invention to provide an electrocoagulation printing ink where non-coagulated ink can easily be removed from the surface of a positive electrode during the printing operation, thereby preventing the generation of undesirable background in a non-image region of the print and improving the image density; a printed matter printed with the ink and free of the declination of the image quality resulted from the undesirable background, with a high level of image density; and an electrocoagulation printing method employing the ink.
We, the inventors of the present invention, have found that both the gradation reproducibility and the image density on a print are largely dependent on the electrocoagulability and the time constant in an electric circuit comprising capacitor (C) and resistor (R) under constant voltage condition, and that an electrocoagulation printing ink has a suitable level of the electrocoagulability corresponding to the energizing rate and is able to produce a desired degree of the gradation reproducibility when a particular relationship between the electrocoagulability and the time constant is satisfied in the electrocoagulation printing ink, so that the electrocoagulation printing ink having both the better gradation reproducibility and the sufficient image density can be realized.
According to one aspect of the present invention there is provided an electrocoagulation printing ink contains an electrolytically coagulable polymer, a soluble electrolyte, a coloring agent, and a liquid medium, in which the electrocoagulability obtained by an electric circuit comprised of capacitor and resistor under constant voltage condition with the time constant of 0.02 seconds and the electrocoagulability obtained with the time constant of 0.09 seconds have a ratio from 1:1.5 to 1:3.0.
It is defined in the above ink that the time constant (T) is a product RC of the capacitance C and the resistance R. The resistance R actually measured in the electrocoagulation printing is formed by filling the electrocoagulation printing ink between the positive and the negative electrodes. When charged from the capacitor, the electrocoagulation printing ink between the positive and negative electrodes is crosslinked to form a coagulum which is hardly soluble in water. It is noted that the electrocoagulability (%) is expressed by [(dry weight of the coagulated ink)/(dry weight of the ink filled between the electrodes)xc3x97100].
The time constant of 0.02 seconds stands for an intermediate grade region of the print and the time constant 0.09 seconds stands for a dark or shadow region near the full black. When the relationship between the electrocoagulability and the time constant mentioned above is satisfied in the electrocoagulation printing ink, the graphic representation of its electrocoagulability versus energizing rate shows a suitable xe2x80x9cslopexe2x80x9d from the shadow region to the intermediate region. Accordingly, the xe2x80x9cslopexe2x80x9d from the highlight to the intermediate also becomes suitable, so that a printed matter with an excellent gradation reproducibility over the image can be provided.
According to another aspect of the present invention there is provided an electrocoagulation printing ink containing at least a bisphenol sulfonic acid polymer as a dispersant, an electrolytically coagulable polymer, a soluble electrolyte, a pigment, and a liquid medium. The bisphenol sulfonic acid polymer can effectively improve the surface property or the dispersibility of the pigment particles contained in the electrocoagulation printing ink, consequently improving both the releasability from the surface of the positive electrode and the image density. That is, the bisphenol sulfonic acid polymer can adhere to the hydrophobic surface of the pigment particles and give a hydrophilic property to the surface of the pigment particles. This allows non-coagulate ink to be released easily from the lipophilic surface of the positive electrode coated with a coating agent containing an oily substance, so that the formation of undesirable background over the print can be prevented. Further, the bisphenol sulfonic acid polymer has a higher stability when adhering to the surface of the pigment particles than any conventional dispersant such as naphthalene sulfonic acid-formaldehyde condensation. This absorption increases electrostatic repulsion between the particles of the pigment by applying negative charges to their surface. Accordingly, the higher dispersing effect can be achieved, thereby encouraging the development of colors and increasing the image density on the print.
According to another aspect of the present invention there is provided an electrocoagulation printing ink containing at least an electrolytically coagulable polymer, a soluble electrolyte, a pigment, and a liquid medium, in which the particle size of the pigment at 50% of the volume accumulation is 0.3 to 2 xcexcm when dispersed in the ink, and a ratio of particles with a particle size equal to or less than 0.2 xcexcm is equal to or less than 25% by weight in the particle size distribution of the pigment. In the electrocoagulation printing ink of the present invention, the particle size of the pigment is controlled in such a given range as explained above, so that the removal or scrape of the non-coagulated ink from the surface of the positive electrode can be facilitated and the formation of undesirable background over the non-image region of the print can be prevented.
A printed matter according to the present invention is printed with the electrocoagulation printing ink of the present invention. By using the electrocoagulation printing ink of the present invention, a printed matter which has an improved gradation reproducibility and a high level of the image quality can be produced. Also, by using the electrocoagulation printing ink of the present invention, it is possible to produce a printed matter with an improved image density and high of quality, without an undesirable background in its non-image region.
An electrocoagulation printing method according to the present invention is implemented using the electrocoagulation printing ink of the present invention, thereby producing a printed matters with high quality.